Radio Frequency (RF) amplifiers are used in a variety of devices, including mobile communications devices such as mobile telephones. In particular, an RF power amplifier is employed to amplify and transmit an RF signal from a mobile communication device.
The required time between battery charges is one important performance specification for a mobile communication device. Meanwhile, an RF power amplifier that transmits the output signal from a mobile communication device often represents the largest power drain on the mobile device's power supply (e.g., a battery). As a result, the required time between charges can be lengthened if the efficiency of the RF power amplifier could be improved.
Therefore, to reduce power consumption, some mobile communication devices exercise some form of control over the output power of the RF power amplifier. In particular, when the mobile communication device is operating somewhat far away from a base station, the device may operate in a “high power” mode wherein the RF output power level is set at or near its maximum value. On the other hand, when the mobile communication device is operating somewhat close to a base station, the device may operate in a “low power” mode wherein the RF output power level is set at a reduced level. Variations of this “dual-mode” operation are possible, for example, where the current battery voltage level and the remaining battery capacity are also taken into consideration when switching between operating modes.
When the RF power amplifier is switched into the “low power” mode, it consumes less current from the battery than when it operates in the “high power” mode. Therefore, when the RF power amplifier is switched to the “low power” mode, the mobile communication device is able to conserve battery power and thereby extend the required time between charges.
However, in a typical arrangement, in the low power mode the operating efficiency of the RF power amplifier is reduced to a significant degree. For the same output power level, when the efficiency of the RF power amplifier is decreased, this means that more wasted current is drawn from the battery. So although the low power mode conserves battery power and thereby extends the required time between charges, it could provide an even greater benefit if the operating efficiency in the low power mode could be improved.
What is needed, therefore, is an improved bias control circuit for an RF amplifier so that it can operate in both a high output power mode and a low output mode with high efficiency. What is further needed is a method of providing bias control to an RF amplifier so that it can operate in both a high output power mode and a low output mode with high efficiency.